GM-CSF is required for optimal expression of collagenase-induced arthritis. Arthritis was initiated in C57BL/6 and GM-CSF-/- mice by intra-articular collagenase injection (Methods) and histology performed 6 weeks later. (A) Representative histologic pictures of knee joints (Safranin O/fast green stain) (original magnification ×100). (B) Quantification of arthritis development (histology) at day 42 (Methods). Results are expressed as the mean ± standard error of the mean; n = 10 mice per strain (from two independent experiments). *P < 0.05, **P < 0.01, ***P < 0.001, GM-CSF-/- versus C57BL/6 mice. C: cartilage; GM-CSF: granulocyte-macrophage colony-stimulating factor; LF: lateral femur; LT: lateral tibia; M: meniscus; MF: medial femur; MT: medial tibia; arrow indicates cartilage damage.